What should resistance readings be for blown drivers?

Dan77

I'm having some trouble with the right hand speaker in particular for my pair of SDA SRS. (It sounds muffled and quiet.) I talked on the phone with the technical support folks, and they suggested I check the tweeters with a multimeter. I did so, and got a reading of 0 Ohms for all of the tweeters, both left and right. I also got a reading of zero for all of the mid-range drivers I tested from the left and right sides. It was my understanding that it should read between 4 and 5, and a reading of 0 indicates a blown driver. Do I have this right, or am I just somehow measuring wrong? Is it possible I'm still getting reasonable sound from the left speaker even if it's blown? (Sorry, totally new to all this!) Any help would be much appreciated.

mhardy6647

"blown" per se would usually mean an open voice coil, which would read infinite (offscale) DC resistance.

Zero ohms would represent a DC short circuit (I am assuming you're using a digital multimeter)... are you reading across the driver's terminals with it disconnected from the crossover? This is the only "right" way to check the driver.

An isolated driver voice coil should read some low but nonzero DC resistance. The actual value will be reasonably close to, but almost certainly not identical to, the nominal (AC) impedance rating of the driver. A dead short across the VC would probably be fatal, too, I suppose... although I don't think it's all that common. Check the flexible "tinsel" leads that go from the terminal to the voice coil on the driver to see if they're touching each other or the metal basket of the driver, or suchlike.

A good 'use test' of a driver is to use a 1.5 volt cell ('flashlight battery') and two clip leads. clip the two clip leads to the two terminals, hold one leg (e.g., the "minus") to the negative end of the battery, and touch the "plus" leg to the positive (button) end of the battery. As the circuit makes, you'll hear a little static-y noise, and the cone should move "out" (not perceptibly for a tweeter). If you hear nothing and/or no cone motion is detected (for a MR driver or woofer, that is) -- then the driver is probably bad. As with the DMM test, this must be done with the "free" driver, removed from the circuit (i.e., disconnected from its crossover).

This low DC voltage, applied for a short time, won't generate enough heat to damage a voice coil in virtually any normal dynamic driver. Don't leave it energized for more than a moment, though, just to be on the safe side. Still, I claim no responsibility for damage, loss of life nor limb! ;- )

Dan77

Thanks very much for this response!

I did, indeed, disconnect the driver coils from the crossover, although I'm actually using an analogue multimeter. (GMT-312 on the OHM X 1K setting - hope that's right?)

Given the zero readings, I take from this that I'm either measuring wrong or there is indeed something wrong with the drivers across the board.

I'll have to postpone the 1.5 volt cell test for tomorrow, but definitely appreciate that suggestion.

mhardy6647

You might want to try "lower" (or is it "higher"?) sensitivity setting if possible.
Your looking for a couple of ohms with a full scale sensitivity of 1000 ohms full scale (although the analog meters usually have a pretty wide 'dynamic range' of readout).

The VCs could be shorted... but that seems awfully odd to me, especially all four of them (or however many there are!).

An analog VOM should read infinite resistance with the two test leads separated in space and zero ohms if the two leads are touched together. Is that indeed the case (i.e., is the VOM working properly)? A meter needs a small DC voltage (supplied by an internal battery) to measure resistance -- is your battery OK?

Dan77

Thanks again for the response.

It looks like the VOM really only has the one setting for ohms, unless I'm misunderstanding.

With the leads separated in space, the needle stays all the way to the left (doesn't move).

I used the ohms adjustment dial to calibrate it so that the needle reads zero (all the way to the right) when the leads are touched together. This is the same 0 reading I get touching the leads to the terminals on the VCs.

I had wondered about the battery as well and was thinking I'd try a new one tomorrow just to be sure.

Schurkey

Apparently the ohmmeter you're using is inappropriate for testing speaker voice coils. A 1K ohm scale is not sensitive enough to react to a voice coil that is probably under 10 ohms.

An analog meter makes this worse. Trying to tell the difference between "0" and "7" on meter intended to show 1000 at full scale won't be discernible to the eye--or--the meter is malfunctioning.

mhardy6647

In fairness -- it does depend on the meter and the scaling; I grew up (literally) with VTVMs that would be more than up to the task of accurately and precisely measuring a couple of ohms... but, yes, in this case, sounds like maybe not the right tool for the job. I am guessing (not to denigrate the OP!) that we're talking about one of those 1000 ohm per volt "pocket tester" type of meters here. I am not 100% sure it wouldn't work adequately to test a loudspeaker VC, but (not to be redundant!) yeah, probably not the best tool for the task.

I'm guessing it be, perhaps, something like this (item "A" in this 1975 R/S catalog page scan) ?


source: www.radioshackcatalogs.com

For the OP's benefit -- and not that this helps at all with the actual question on the table :-P -- a good-enough DMM is inexpensive and easily obtained. Probably well worth the investment for even a "once in a while" troubleshooter. You don't need a multi-hundred dollar Fluke (although... one does get what one pays for); something like this would probably be fine.

http://www.parts-express.com/digital-multimeter-dmm-with-convenient-tilt-display--390-545

For a good old-fashioned VOM, though, hard to beat an (old-school) Simpson or Triplett.

DSCN5602 by Mark Hardy, on Flickr

.... and let's not even get started on VTVMs ;- )

Dan77

That is a gorgeous VOM. I just have this $18 one I picked up at the hardware store.



In the meantime, I ran the speakers with all the tweeters removed on both sides. At the very least, I have more than one problem. When I balance the sound all the way right, I'm not getting much from the right speaker, but I do seem to be getting input back to the left speaker through the interconnect cable. Problem with the crossover?

In any case, I still want to run this resistance testing to the ground, especially since there is some brown discoloration inside the tweeter cones. Unfortunately, it's going to have to wait a as I head home for a few days. I'll get started again on Sunday, possibly with new digital VOM and battery.

Thanks for all the feedback, and happy holidays!

mhardy6647

yeah, get yourself a nice(r) one; too late to ask for one for Christmas? ;- )
That said, note that the resistance scale provides plenty of resolution down at the low end -- i.e., it may well be telling you the truth.

I should take a better photo of that Simpson; it is indeed a nice meter. Even more so given its provenance (from the "Yankee Swap Pile" at our erstwhile town 'transfer station' - dump - in Harvard, MA).

Best of luck with your loudspeakers; there are plenty of folks here with much product-specific knowledge that can help you troubleshoot (I am not one of them, unfortunately).


Have a great holiday!

skrol

Very nice Simpson. Dan77's meter looks like a knock-off of my little vintage Triplett 310.

Just a note... I have found that inexpensive DVMs can be very useful for general purpose, however, when measuring at the more extremes (ie. <10 ohms) they can be quite inaccurate.

Measuring a 0.5 ohm 1% resistor with my few $ Harbor Freight DVM indicated 1.2 ohms vs my Fluke 187 which read 0.57 ohms.

You can find good deals on Ebay for decent meters if you are careful. I paid $130 for my Fluke 187 and it came with probes and a nice leather case. It still has the clear protector over the display and had been calibrated.